National Instruments NI-Motion - Manual (Page 37)

Chapter 3 Tuning Servo Systems
© National Instruments Corporation 3-9 NI-Motion User Manual
Velocity Feedback
You can configure the NI motion controller for velocity feedback using the
Kv (velocity feedback) gain. Using Kv creates a minor velocity feedback
loop. This is very similar to the traditional analog servo control method of
using a tachometer for closing the velocity loop. This type of feedback is
necessary for systems where precise speed control is essential.
You can use a less expensive standard torque, or current mode, amplifier
with the velocity feedback loop on NI motion controllers to achieve the
same results you would get from using velocity amplifiers, as shown in
Figure 3-5.
Figure 3-5. Velocity Feedback
Setting any non-zero value for Kv allows you to use the Kv term instead of
or in addition to the Kd term to stabilize the system.
Velocity feedback gain (Kv) is similar to derivative gain (Kd) except that it
scales the velocity estimated from encoder resources only. The derivative
gain scales the derivative of the position error, which is the difference
between the instantaneous trajectory position and the primary feedback
position. Like the Kd term, the velocity feedback derivative is calculated
every derivative sample period, and the contribution is updated every PID
sample period, as shown in Figure 3-6.
Σ Σ